1. Field of the Invention
The invention relates generally to switching power supplies and, more particularly, the invention relates to a power factor compensation controller for use in switching power supplies.
2. Description of Related Technology
Generally speaking, a switching power supply (SPS) provides a cost effective and energy efficient device for converting energy from a single direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the supply voltage. Traditionally, a SPS has an integrated control circuit that modulates the duty cycle of a transistor switch, which controls the flow of energy into the primary of a transformer or an input of an inductor to produce one or more desired output voltages that are derived from the secondary of the transformer or the output of the inductor, respectively. As is well known, the energy (i.e., the time integral of power) supplied to the primary of the transformer or the inductor minus efficiency losses equals the energy transferred to the secondary of the transformer or the output of the inductor. Thus, if more energy is needed at an output of the SPS, then the control circuit increases the duty cycle of the transistor switch to provide more energy to the primary of the transformer or the input of the inductor. Conversely, if less energy is needed at the output of the SPS, then the control circuit decreases the duty cycle of the transistor switch.
As is also well known, the inductive components within a SPS may be operated using one of a variety of conventional current conduction modes depending on the particular application for the SPS. For example, a boundary conduction mode (BCM) and a discontinuous conduction mode (DCM) each result in large peak currents and, as a result, BCM and DCM are typically used where small value inductors are desirable and where the SPS has a relatively small load (i.e., low power output). On the other hand, a continuous conduction mode (CCM) control technique requires a relatively large inductance value, which reduces the peak-to-average current ratio within the SPS and which allows a SPS to operate at relatively high power levels (e.g., 300 watts).
The power factor of a SPS can have a significant impact on the efficiency with which energy is conveyed from a source of input power (e.g., a source of alternating current line voltage) to a load that is connected to an output of the SPS. As a result, a variety of conventional techniques for controlling the power factor of a SPS operating in one of the aforementioned current control modes have been developed. For example, control over the power factor of a boost convertor operating in a CCM may be accomplished using an average current mode control (ACMC) method, a charge control method, a peak current mode control (PCMC) method, or a hysteresis control method. While the conventional ACMC, PCMC, hysteresis control, and charge control methods may be used to compensate the power factor of a SPS, these conventional methods typically require relatively complex circuitry and, in some cases, these conventional methods provide a limited compensation range that fails to compensate the power factor of the SPS over the entire range of SPS output loads.